First of all, it’s not shocking to find out that rooms with no sunlight produced negligible energy during that time. When you think about it, if they had been gathering a statistically significant amount wouldn’t that mean the lighting used in those rooms was incredibly inefficient? In other words, there’s no way you need to be making that much light.

But he did find that proper positioning in rooms that catch sunlight during the day can result in usable energy for small loads. He established that a 0.5 Watt panel harvested just a bit more than half of what a 1 Watt panel did. But perhaps the most useful discovery was that it’s quite a bit more efficient to have a charging circuit store energy in a battery rather than directly powering a fixed load.

It will take us a few more viewings to really decide what we can take away from the experiment for our own projects. But we appreciate [Mathieu’s] quest for knowledge and his decision to put this information out there so that others can learn from it.

19 thoughts on “Reviewing the numbers from one month of solar harvesting”

” it’s quite a bit more efficient to have a charging circuit store energy in a battery rather than directly powering a fixed load.”
I didn’t see that in the article. He did suggest using a big battery to store energy from the better months.
However, solar cell efficiency is partly dependent on the voltage maintained across the cell. An optimal load maintains an ideal voltage at the cell. So, a good solar-optimised regulator/charger would indeed yield more energy than most loads directly connected to a cell/panel.

Of course, if one takes onto account all the embodied energy that went into the extraction of the metals for your generator, and extracting the petroleum/transporting/refining/transporting the refined gasoline to power the generator, your premise FAILS miserably. And lets not even mention the carbon footprint of your faulty comparison.

Ha, ha, ha, you funny guy.

Dax sez:
>
> It costs between 80-300 kg of CO2 to produce a square meter of solar panel, so it makes very little sense to put any on small gadgets and especially indoors.
>
> It would be polluting more than using a tiny gasoline powered generator to charge up your cellphone.

Gasoline has an embodied energy cost of roughly 1.25 times its energy contents. To turn it into electricity you need four times as much gasoline, which means 5 times the energy.

A kilogram of gasoline produces roughly 3.2 kg of CO2 and contains 8.76 kWh of energy, therefore you produce 1800 grams of CO2 for every kWh of electricity produced.

Then let’s take a solar panel. Let’s take the best case scenario and say that it only produces 80 kilograms to manufacture it. We take a 10×10 cm square, or 1/100 of it, which is worth 800 grams of CO2. Then we use it indoors to produce roughly 20 mWh per year for 20 years. That’s a total of 0.4 Watt-hours, or 0.0004 kWh, or adjusted to the same units: 2 million grams of CO2 per kWh.

1800 g/kWh is so much less than 2,000,000 that we don’t even have to take into account how much energy went into producing the tiny generator. Any way you make it, it’s going to be more ecological to simply not manufacture the solar panel by a factor of a thousand.

This article kind of proves the obvious: there’s no place for solar cells inside.

I don’t really see why it could be feasible to charge the battery more in sunnier months and use the energy in darker ones because: batteries have discharge rate that might be comparable to the extra charge in summer and if you are getting so little energy(at this scale) isn’t it cheaper to use alkaline batteries instead of solar panel+rechargeable? the cost of the solar version might be equal to enough alkaline batteries to power the application throughout it’s life.

Example: the 1W panel extracts 50uWh per day. That’s 18mWh per year. An AA cell has about 3Wh(1.5V * 2Ah). Even if you get 10X more in summer, it is still more than 10 times less than an AA battery.

Yep, but it had to “proven”. The thing is that you don’t find people doing the same kind of experiments on the internet, so you can’t really know for sure that small solar panels are completely useless.
However, for a small sensor network, that might just do the trick. In my opinion, li-ion discharge rate is not that big.

Having a quick look at http://www.physibel.be/voltra_sol2.htm gave some interesting ideas. An average sized room with an average sized window looks to give approx 5W solar energy per square meter on a sunny day.

Given that a 0.5W panel is only 55m x 70mm, and efficiencies are around 15% it is not unreasonable to expect that at most 3mW that can be generated during sunny times, and looking at the graphs that was only on day in three!

When you allow for the aspect of the window (e.g. south facing) you might only get that level for a couple of hours a day, making your 1W solar cell average under a mW per day….

Here’s the thing though, solar panels are awful at collecting light from off angles. They only “see” the light that’s coming more or less straight at them, which is a tiny portion of the available ambient light.

i’ve often wondered if it would be feasible to use the magnifying glass effect on a solar panel. basically using a few lenses and a prism to collect and then scatter the light.. assuming of course just focusing the ant-ray of doom on one directly would burn out the collector

I agree, but we all need to understand that adding Solar to their home is an asset that should boost the actual valuation of their property if / when they make a choice to sell. With the environment the way it is going we are unable to overlook any product or service that provides zero cost power at no cost to both the consumer and more significantly the earth!